Clear powder coating

ABSTRACT

The present invention relates to a transparent powder coating material comprising 
     a) at least one epoxy-containing binder with a content of from 20 to 45% by weight, preferably from 25 to 40% by weight of glycidyl-containing monomers and with or without a content of vinylaromatic compounds, preferably styrene, 
     b) tris(alkoxycarbonylamino)triazine and polycarboxylic acids, preferably straight-chain aliphatic dicarboxylic acids and/or carboxy-functional polyesters, as crosslinking agents, and 
     c) if desired, catalysts, auxiliaries, additives typical for transparent powder coating materials, such as degassing agents, levelling agents, UV absorbers, free-radical scavengers, antioxidants.

The present invention relates to a transparent powder coating materialwhich is particularly suitable as a coating for car bodies that arecoated with waterborne coating material.

At present, liquid coating materials are preferably used for the coatingof car bodies. These coating materials cause numerous environmentalproblems owing to their solvent content. This is true even when aqueouscoating materials are employed.

For this reason, increased efforts have been made in recent years to usepowder coating materials for the coating. The results to date, however,have been unsatisfactory; in particular, transparent powder coatingmaterials still exhibit weaknesses in terms of chemical resistance andyellowing. Epoxy/carboxy-crosslinked transparent powder coatings exhibita markedly poorer etch resistance relative to water, tree resin andsulphuric acid.

The object of the present invention is to provide a transparent powdercoating material which relative to transparent powder coating materialsknown to date exhibits markedly better etch resistance and a reducedtendency towards yellowing after crosslinking. In this context, thetransparent powder coating material should be able to be supplied forapplication in solid form.

This object is achieved by a transparent powder coating materialcomprising

a) at least one epoxy-containing binder with or without a content offrom 20 to 45% of glycidyl-containing monomers and with or without acontent of vinylaromatic compounds, preferably styrene,

b) tris(alkoxycarbonylamino)triazine and polycarboxylic acids,preferably straight-chain aliphatic dicarboxylic acids and/orcarboxy-functional polyesters, as crosslinking agents, and

c) if desired, catalysts, auxiliaries, additives typical for transparentpowder coating materials, such as degassing agents, levelling agents, UVabsorbers, free-radical scavengers, antioxidants.

The content of glycidyl-containing monomers is preferably from 25 to45%. Particular preference is given to from 30 to 45% by weight, inparticular from 30 to 40%. From 30 to 35% are very particularlypreferred. Utmost preference is given to from 26 to 35%, especially from27 to 33%.

The following proportions are preferred in this context:

a) 60-80 parts

b) 15-30 parts

c) 3-10 parts

A suitable epoxy-functional binder for the solid transparent powdercoating material comprises, for example, epoxy-functional polyacrylateresins which can be prepared by copolymerizing at least oneethylenically unsaturated monomer which contains at least one epoxygroup in the molecule with at least one further ethylenicallyunsaturated monomer which contains no epoxy group in the molecule, atleast one of the monomers being an ester of acrylic acid or methacrylicacid. Epoxy-functional polyacrylate resins of this kind are known, forexample, from EP-A-299 420, DE-B-22 14 650, DE-B-27 49 576, U.S. Pat.No. 4,091,048 and U.S. Pat. No. 3,781,379.

Examples of ethylenically unsaturated monomers which contain no epoxygroup in the molecule are alkyl esters of acrylic and methacrylic acidcontaining 1 to 20 carbon atoms in the alkyl radical, especially methylacrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butylacrylate, butyl methacrylate, 2-ethylhexyl acrylate and 2-ethylhexylmethacrylate. Further examples of ethylenically unsaturated monomerswhich contain no epoxy groups in the molecule are acid amides, such asacrylamide and methacrylamide, vinylaromatic compounds, such as styrene,methylstyrene and vinyltoluene, nitrites, such as acrylonitrile andmethacrylonitrile, vinyl halides and vinylidene halides, such as vinylchloride and vinylidene fluoride, vinyl esters, such as vinyl acetate,and hydroxyl-containing monomers, such as hydroxyethyl acrylate andhydroxyethyl methacrylate, for example.

The epoxy-functional monomers employed in the epoxy-functional bindersare preferably glycidyl acrylate, glycidyl methacrylate and allylglycidyl ether. The epoxy-functional polyacrylate resin normally has anepoxide equivalent weight of from 300 to 2500, preferably from 420 to700, a number-average molecular weight (determined by gel permeationchromatography using a polystyrene standard) of from 2000 to 20,000,preferably from 3000 to 10,000, and a glass transition temperature(T_(g)) of from 30 to 80, preferably from 40 to 70 and, with particularpreference, from 40 to 60° C. (measured with the aid of DifferentialScanning Calorimetry (DSC)). Very particular preference is given toabout 50° C. Mixtures of two or more acrylate resins can also beemployed.

The epoxy-functional polyacrylate resin can be prepared by common andwell-known methods, by addition polymerization.

As component (b), tris(alkoxycarbonylamino)triazines in accordance withU.S. Pat. No. 4,939,213, U.S. Pat. No. 5,084,541 and EP 0 624 577 areemployed. Such compounds are also known from DE 2509561 and also PatentAbstracts of Japan, Publ. No. 09194769 A. However, thetris(alkoxycarbonylamino)triazines are employed therein only forcationic electrodeposition coating materials.

In accordance with the invention, the tris(alkoxycarbonylamino)triazinesare of the formula

in which R=methyl, butyl, ethylhexyl groups. Derivatives of thesecompounds can also be employed.

The invention prefers the methyl/butyl mixed esters. These have theadvantage over the plain methyl esters of better solubility in polymermelts, and butyl-ethylhexyl mixed esters. The plain butyl esters arealso preferred in accordance with the invention.

The tris(alkoxycarbonylamino)triazines and their derivatives can also,in accordance with the invention, be employed in a mixture withconventional crosslinking agents (component C). Here, blockedpolyisocyanates different from the tris(alkoxycarbonyl amino)triazinesare particularly suitable. Similarly, it is possible to employ aminoresins, e.g. melamines. The tris(alkoxycarbonylamino)triazines can bepresent in amounts of from 1 to 10, preferably from 2 to 10% by weight.

In principle it is possible to employ any amino resin suitable fortransparent topcoats, or a mixture of such amino resins.

Resins of this kind are well known to the person skilled in the art andare offered as commercial products by numerous companies. Amino resinsare condensation products of aldehydes, especially formaldehyde, and,for example, urea, melamine, guanamine and benzoguanamine. The aminoresins contain alcohol groups, preferably methylol groups, some or allof which in general are etherified with alcohols.

Further suitable crosslinkers are carboxylic acids, especiallysaturated, straight-chain, aliphatic dicarboxylic acids having 3 to 20carbon atoms in the molecule. It is very particularly preferred toemploy dodecane-1,12-dioic acid. In order to modify the properties ofthe finished transparent powder coating materials, othercarboxyl-containing crosslinkers may be employed if desired. As examplesof these mention may be made of saturated branched or unsaturatedstraight-chain di- and polycarboxylic acids and of polymers havingcarboxyl groups.

Suitability extends to transparent powder coating materials whichcomprise an epoxy-functional crosslinker and an acid-functional binder.

Examples of suitable acid-functional binders are acidic polyacrylateresins which can be prepared by copolymerizing at least oneethylenically unsaturated monomer containing at least one acid group inthe molecule with at least one further ethylenically unsaturated monomercontaining no acid group in the molecule.

The epoxy-functional binder and the carboxyl-functional crosslinker and,respectively, the carboxyl binder and epoxy crosslinker are normallyemployed in an amount such that there are from 0.5 to 1.5, preferablyfrom 0.75 to 1.25, equivalents of carboxyl groups per equivalent ofepoxy groups. The amount of carboxyl groups present can be determined bytitration with an alcoholic KOH solution.

In accordance with the invention the binder includes vinylaromaticcompounds, especially styrene. In order to limit the risk of cracking onexposure to weathering, however, their content is not more than 35% byweight. Preference is given to from 10 to 25% by weight.

If desired, the solid powder coating materials include one or moresuitable catalysts for curing the epoxy resin. Suitable catalysts arephosphonium salts of organic or inorganic acids, quaternary ammoniumcompounds, amines, imidazole and imidazole derivatives. The catalystsare generally employed in proportions of from 0.001% to about 2% byweight, based on the overall weight of the epoxy resin and of thecrosslinking agent.

Examples of suitable phosphonium catalysts are ethyltriphenylphosphoniumiodide, ethyltriphenylphosphonium chloride, ethyltriphenylphosphoniumthio-cyanate, ethyltriphenylphosphonium acetate-acetic acid complex,tetrabutylphosphonium iodide, tetrabutylphosphonium bromide andtetrabutylphosphonium acetate-acetic acid complex. These and othersuitable phosphonium catalysts are described, for example, in U.S. Pat.No. 3,477,990 and U.S. Pat. No. 3,341,580.

Examples of suitable imidazole catalysts are 2-styrylimidazole,1-benzyl-2-methylimidazole, 2-methylimidazole and 2-butylimidazole.These and other imidazole catalysts are described, for example, inBelgian Patent No. 756,693.

In addition, the solid powder coating materials may also, if desired,comprise auxiliaries and additives. Examples thereof are levellingagents, antioxidants, UV absorbers, free-radical scavengers, flow aidsand degassing agents, such as benzoin, for example.

The solid powder coating materials are prepared by known methods (cf.e.g. product information from BASF Lacke+Farben AG, “Pulverlacke”[powder coatings], 1990) by homogenization and dispersion by means, forexample, of an extruder, screw kneading apparatus and the like.Following preparation of the powder coating materials, they are preparedfor dispersion by grinding and, if desired, by classifying and sieving.

The average particle size of the powder coating materials of theinvention lies between 1 and 25 μm, preferably below 20 μm and, withparticular preference, from 2 to 10 μm.

The powder coating materials can be applied by the methods known fromthe prior art. In this context, use in the form of transparent coatingmaterials is also suitable.

Stoving can be carried out at temperatures of as low as 130° C. It ispossible to carry out stoving at from 130 to 180° C., preferably from135 to 155° C.

It is surprising that with the crosslinker employed in accordance withthe invention the etch resistance to water, tree resin and sulphuricacid is improved and the tendency towards yellowing is significantlyreduced.

In the text below, the invention is described in more detail withreference to the examples:

1. PREPARING THE ACRYLATE RESIN

21.1 parts of xylene are introduced into a vessel and are heated to 130°C. The following components are metered into the initial charge at 130°C. over the course of 4 h by way of two separate feed containers:initiator: 4.5 parts of TBPEH (tert-butyl perethylhexanoate) mixed with4.86 parts of xylene, and monomers: 10.78 parts of methyl methacrylate,25.5 parts of n-butyl methacrylate, 17.39 parts of styrene and 23.95parts of glycidyl methacrylate. The mixture is subsequently heated to180° C. and the solvent is stripped off in vacuo <100 mbar.

2. PREPARING THE TRANSPARENT POWDER COATING MATERIAL COMPARATIVE EXAMPLE

77.5 parts of acrylate resin, 18.8 parts of dodecanedioic acid, 2 partsof Tinuvin 1130 (UV absorber), 0.9 part of Tinuvin 144 (HALS), 0.4 partof Additol XL 490 (levelling agent) and 0.4 part of benzoin (degassingagent) are intimately mixed in a Henschel fluid mixer, the mixture isextruded on a BUSS PLK 46 extruder, the extrudate is ground in aHosokawa ACM 2 mill, and the ground material is screened through a 125μm sieve.

3. EXAMPLE 1

73.5 parts of acrylic resin, 17.8 parts of dodecanedioic acid, 5.0 partsof tris(alkoxycarbonylamino)triazine, 2 parts of Tinuvin 1130 (UVabsorber), 0.9 part of Tinuvin 144 (HALS), 0.4 part of Additol XL 490(levelling agent) and 0.4 part of benzoin (degassing agent) areintimately mixed in a Henschel fluid mixer, the mixture is extruded on aBUSS PLK 46 extruder, the extrudate is ground in a Hosokawa ACM 2 mill,and the ground material is screened through a 125 μm sieve.

4. APPLYING THE TRANSPARENT POWDER COATING MATERIAL

The transparent powder coating material is applied by means ofelectrostatic spray gun to steel panels coated with customary commercialelectrodeposition coating material and black waterborne coatingmaterial. The panels are subsequently stoved for 30 minutes at atemperature of 140° C.

5. TESTING THE CHEMICAL RESISTANCE OF THE TRANSPARENT POWDER COATINGMATERIALS

Tree resin (0.025 ml) and 1% strength H₂SO₄ (0.025 ml) are applied tothe test panels, and the panels are subsequently subjected to the actionof a gradient oven (from Byk) for 30 minutes. A note is made of thetemperature at which initial damage occurs.

Tree resin H₂SO₄ Comparative 37° C. 47° C. Example Inventive 53° C. 51°C. Example

What is claimed is:
 1. A transparent powder coating material comprisinga) at least one epoxy-containing binder with a content of from 20 to 45%by weight of glycidyl-containing monomers, b) a crosslinking agentcomprising from 1% to 10% by weight of a member selected from the groupconsisting of tris(alkoxycarbonylamino)triazines, derivatives oftris(alkoxycarbonylamino)triazines, and mixtures thereof, and furthercomprising at least one member selected from the group consisting ofpolycarboxylic acids, straight chain aliphatic dicarboxylic acids,carboxy-functional polyesters, and mixtures thereof, and c) optionally,one or more members selected from the group consisting of catalysts,auxiliaries, additives, degassing agents, leveling agents, UV absorbers,free-radical scavengers, antioxidants, and mixtures thereof.
 2. Thetransparent powder coating material of claim 1, comprising from 25 to45% by weight of glycidyl-containing monomers.
 3. The transparent powdercoating material of claim 2, comprising from 30 to 45% by weight ofglycidyl-containing monomers.
 4. The transparent powder coating materialof claim 3, comprising from 30 to 35% by weight of glycidyl-containingmonomers.
 5. The transparent powder coating material of claim 1, whereincomponent (a) further comprises one or more vinylaromatic compounds. 6.The transparent powder coating material of claim 1, wherein component(a) comprises an epoxy-functional polyacrylate resin and theepoxy-functional monomers are selected from the group consisting ofglycidyl acrylate, glycidyl methacrylate, allyl glycidyl ether, andmixtures thereof.
 7. An aqueous dispersion, comprising a component A inthe form of a solid powder coating, said component A comprising thetransparent powder coating material of claim 1, and an aqueous componentB.
 8. The aqueous dispersion of claim 7, wherein aqueous component Bcomprises a) at least one nonionic thickener b) optionally, one or moremembers selected from the group consisting of catalysts, auxiliaries,defoamers, wetting agents, dispersing auxiliaries, carboxy-functionaldispersants, antioxidants, UV absorbers, free-radical scavengers,biocides, solvent, leveling agents, neutralizing agents, amines, waterretention agents, and mixtures thereof; and wherein the at least oneepoxy-containing binder has a content of from 20% to less than 25% byweight of glycidyl containing monomers.
 9. The aqueous dispersion ofclaim 7, having a pH of from 4.0 and 7.0, and wherein the at least oneepoxy-containing binder has a content of from 20% to less than 25% byweight of glycidyl containing monomers.
 10. The transparent powdercoating material of claim 1 having a particle size of not more than 20μm.
 11. A method for coating uncoated and coated car bodies made frommetal sheet and/or plastic by means of electrostatically assistedhigh-speed rotation or pneumatic application, comprising applying to asubstrate the transparent powder coating material of claim
 1. 12. Thetransparent powder coating material of claim 5, wherein component (a)comprises not more than 35 weight percent of one or more vinylaromaticcompounds, based on component a).
 13. The aqueous dispersion of claim 9having a pH of from 5.5 and 6.5.
 14. The transparent powder coatingmaterial of claim 10 having a particle size of from 2 to 10 μm.